With initial reference to FIGS. 1 and 2, a recirculating planar magnetron (RPM) 100 is a high-power crossed-field radio frequency (RF) or microwave source that includes a cathode 102 surrounded by an anode 104 for creating a direct current (DC) or quasi-DC electric field (E) applied from the cathode to the anode region. Additionally, magnetic elements 106 are placed on either side of the cathode 102 and anode 104 for creating a magnetic field (B) that is orthogonal to the electric field. Accordingly, as the term is used herein, “crossed field” refers to the fact that a static magnetic field (B) applied to the device and the direct current (DC) or quasi-DC electric field (E) applied from the cathode to the anode regions of the device are generally orthogonal in direction. Microwaves generated within the RPM 100 are directed away from the anode 104 via waveguides 124 to the intended load.
As shown in FIG. 3, the anode 104 includes first and second planar magnetron sections 108, 110, respectively (also referred to herein as the “upper” and “lower” magnetron sections, respectively, as illustrated). Additionally, recirculation sections 112 connect the planar magnetron sections 108, 110 together. Each of the planar magnetron sections 108, 110 contains periodic corrugations comprised of vanes 114 and cavities 116. A “period” of the planar magnetron section is the space or distance required for a structure of the planar magnetron to be repeated. In this case, the period length is equal to the width of one vane 114 and one cavity 116. In this embodiment, the same number of vanes 114 and cavities 116 are placed on each of the planar magnetron sections 108, 110. However, in other embodiments, the number of vanes and cavities on one planar magnetron section may be different from the number of vanes and cavities on the opposite planar magnetron section. This structure of vanes 114 and cavities 116 is often referred to as a “slow wave structure” because of its tendency to slow the velocity of oscillatory electromagnetic (or “EM”) waves traveling along the structure to less than the speed of light, which is necessary for the RPM 100 to function.
In FIGS. 4A and 4B, a cross section of a cathode positioned between the first and second planar magnetron sections 108, 110 of the anode 104 is provided. In these images, the recirculation sections have been removed for simplicity. In FIG. 4A, a solid cathode 102′ is shown. On the other hand, in FIG. 4B, a segmented cathode 102 having a number of gaps 118 provided along its length is shown. The gaps 118 formed in the segmented cathode 102 are laterally aligned with the cavities 116 formed between the vanes 114 of the upper and lower planar magnetron sections 108, 110. In each case, the electric field (E) and magnetic field (B) are illustrated. The cross product of these two fields results in electrons drifting in the direction shown by VExB along both sides of the cathode and then around the recirculation sections 112, which results in the creation of microwaves being formed in the cavities 116 of the anode 104.
In RPMs utilizing a solid cathode 102′ (FIG. 4A), the first and second planar magnetron sections 108, 110 couple primarily through the recirculation sections located adjacent each of the ends of the cathode. Coupling in this manner can be somewhat weak. As a result, during operation of this type of RPM, it is possible for the electromagnetic RF oscillations in the upper and lower planar magnetron sections 108, 110 to drift in frequency and phase with respect to one another. On the other hand, as shown in FIG. 4B, a segmented or “mode control” cathode 102 provides gaps 118 that enable additional coupling paths to be created between the upper and lower planar magnetron sections 108, 110. Use of a mode control cathode 102 results in better frequency locking and phase stability of the overall device.
One method for extracting microwave power from an RPM requires two adjacent cavities of a magnetron slow wave structure to be coupled together into a single extraction waveguide. To function optimally while utilizing this extraction method, the RPM should operate in “pi mode,” which occurs when the RF electric field across each adjacent magnetron cavity differs by 180 degrees (or pi radians). When using a mode control cathode, its modes of operation may be divided into a set of even modes and a set of odd modes and each set of modes has its own pure pi mode (i.e., the set of odd modes will have an odd pi mode and the set of even modes will have an even pi mode). The terms “even” and “odd” are used to denote when the RF oscillations of the upper and lower sections of the RPM are in phase or 180 degrees out of phase, respectively. Because power coupled into waveguides attached to the upper and lower sections of the RPM will be dictated by this phase relationship, the RF energy in the extraction waveguides coupled to the upper RPM section may either be in phase or 180 degrees out of phase with the RF energy in the extraction waveguides coupled to the lower RPM section. As discussed above, the gaps 118 formed in the segmented cathode 102 are laterally aligned with the cavities 116 formed between the vanes 114 of the upper and lower planar magnetron sections 108, 110. This configuration allows the required electromagnetic communication to occur between the upper and lower slow wave structures that allows the device to operate in either the odd pi mode or the even pi mode.
The dispersion diagram shown in FIG. 5 describes the relationship between frequency and phase shift per cavity for a given set of modes. Plotted in the dispersion diagram are curves that represent the frequency and phase shift relationships for the first passband of the even mode 136 and odd mode 138 that occur when a mode control cathode is used. As illustrated by the dispersion diagram, when using a mode control cathode, the strong coupling between upper and lower planar magnetron sections allowed by the gaps in the cathode enhances phase synchronism in the electromagnetic oscillations of the upper and lower planar magnetron sections. As discussed above, use of the mode control cathode splits the first passband into a set of even modes and a set of odd modes. If a solid cathode were to be used, there would be only one curve. The pi mode of each mode set occurs where the phase shift per cavity is equal to 180 degrees or pi radians.
Cross-sectional views of an RPM operating in pi mode are illustrated in FIGS. 6A-6C. In FIG. 6A, an RPM with a solid cathode 102′ is illustrated. Coupling of the upper and lower planar magnetron sections 108, 110 of the RPM takes place primarily through the recirculation sections (not shown), as discussed above. The coupling is generally weak and, as a result of this weak coupling, the pi mode oscillations in the upper and lower planar magnetron sections 108, 110 may not be completely synchronized in phase. The upper electron spokes 120 are in a slightly different configuration from the lower electron spokes 122. This indicates that the pi mode oscillations in the upper planar magnetron section 108 are slightly out of phase with the pi mode oscillations in the lower planar magnetron section 110 and, thus, have a different appearance.
On the other hand, FIG. 6B depicts an RPM with a mode control cathode 102 operating in the even pi mode and FIG. 6C depicts an RPM with a mode control cathode operating in the odd pi mode. In this case, the upper and lower electron spokes 120, 122 are significantly more uniform in appearance and shape due to the increased coupling that is made possible by the gaps 118 formed in the segmented cathode 102. The uniformity in shape is an indicator that the upper and lower slow wave structures 108, 110 are locked into the same mode (either the even or odd pi mode) instead of operating in different modes.
Representations of the oscillatory RF electric field 126 for the even pi mode and the odd pi mode are shown in FIGS. 7A and 7B, respectively. The relative polarity of each vane at the depicted instant of time is indicated by the “+” or “−”. The RPMs illustrated are operating in the pi mode and, for that reason, adjacent vanes in the upper and lower planar magnetron sections 108, 110 have polarities that differ by 180 degrees (or pi radians). Thus, a “period” of the EM emission in the pi mode is the amount of space required for the polarity to be repeated. For example, a period is the distance from one vane having a relative polarity of “+” to the next vane having a relative polarity of “+”. Likewise, a period is also the distance from one vane having “−” relative polarity to the next vane having a “−” relative polarity. In this case, when the RPM is operating in pi mode, the period length is equal to the width of two vanes and two cavities. In this embodiment, the same number of vanes and cavities are placed on each of the planar magnetron sections 108, 110. During even pi mode operation (FIG. 7A), opposing vanes in the upper planar magnetron 108 section and lower planar magnetron section 110 have the same polarity. For example, vane 114A and vane 114B each have a matching “+” polarity. By contrast, during odd pi mode operation (FIG. 7B), opposing vanes 114 in the upper planar magnetron section 108 and lower planar magnetron section 110 have opposite polarity. For example, vane 114C has a “+” polarity, but vane 114D has a “−” polarity.
In FIG. 8, applied magnetic field is plotted against planar diode voltage (related to the electric field E applied from the cathode 102 to the anode 104 in FIG. 4B) for various modes of RPM operation. The “planar diode voltage” refers to the DC or quasi-DC voltage applied between the cathode and the anode of the RPM. Application of this voltage allows the formation of the DC or quasi-DC electric field E discussed previously. As explained below, this plot demonstrates a number of advantages associated with operating an RPM in the even pi mode over operating the RPM in the odd pi mode.
First, the even pi mode is attainable at lower magnetic fields than is the odd pi mode. This means that an RPM intended to operate in the even pi mode requires less power for electromagnetics or less magnetic material for permanent magnets than would an RPM intended to operate in the odd pi mode. Second, the magnetic field range over which the even pi mode is accessible is much larger than the range over which the odd pi mode is accessible. This means that the system controlling the magnetic field setting for an RPM intended to operate in the even pi mode would require a lower degree of precision than the magnetic field control system for an RPM intended to operate in the odd pi mode. A lower precision control system could be expected to be lower in cost and complexity than one required to provide a higher degree of precision. Lastly, the range of applied magnetic field magnitudes in which the odd pi mode can be accessed also supports undesirable modes, such as the
      7    ⁢                  ⁢    π    6mode. This indicates that the operation of the odd pi mode is likely to be less stable than that of the even pi mode. This reduced stability could allow the RPM to more easily start up in a mode other than the odd pi mode or to uncontrollably transition to another mode when originally operating in the odd pi mode.
On the other hand, there are also advantages to operating an RPM in the odd pi mode over operating the RPM in the even pi mode. FIG. 9A illustrates an extracted RPM, where pairs of adjacent cavities 116 are coupled into waveguides 124 via apertures 128 in the cavity walls for extracting microwave energy from the anode 104. The RPM is provided with a mode control cathode 102 and is operating in the even pi mode. As a consequence of operating in the even pi mode, the EM waves 130 within the upper extraction waveguides 124 at this particular instant in time are oriented in a downward direction, whereas the EM waves 132 within the lower extraction waveguides 124 at this particular instant in time are oriented in an upward direction. Thus, EM waves 130 are 180 degrees out of phase with EM waves 132. In this scenario, if the upper and lower waveguides 124 are brought together, the EM waves 130, 132 would partially or completely cancel each other out (i.e., destructively interfere), thereby resulting in an inability to efficiently extract power from the device. This effect is depicted in FIG. 10A, where the EM waves 130, 132 in the upper and lower waveguides 124 are 180 degrees out of phase, which results in no transmitted power in the combined output waveguide 134.
On the other hand, FIG. 9B illustrates an extracted RPM provided with a mode control cathode 102 that is operating in the odd pi mode. As a consequence of operating in the odd pi mode, the EM waves 130 within the upper extraction waveguides 124 at this particular instant in time are oriented in a downward direction, and the EM waves 132 within the lower extraction waveguides 124 at this particular instant in time are also oriented in a downward direction. Thus, EM waves 130 are in phase with EM waves 132. If the upper and lower waveguides are brought together, the waves would combine constructively (i.e., constructive interference) into a combined wave, thereby resulting in efficient power extraction from the device. This effect is depicted in FIG. 10B, where the EM waves 130, 132 in the upper and lower waveguides 124 are in phase and combine constructively to generate combined wave 140. This results in the transmittal of power in the combined output waveguide 134.
As shown above, operating an RPM in the even pi mode is advantageous because it results in increased stability and reduced requirements for the applied magnetic field. However, operation in the odd pi mode is also advantageous because it results in in-phase RF extraction of power from the RPM. Accordingly, what is needed is a recirculating planar magnetron that provides the desirable phase relationship between extracted microwave power in the upper and lower waveguides which is characteristic of operating in the odd pi mode, while also minimizing power requirements and precision control necessary to operate as characterized by operating in the even pi mode.